block/qcow2: refactor encryption code

block/qcow2: refactor encryption code

* Change the qcow2_co_{encrypt|decrypt} to just receive full host and
  guest offsets and use this function directly instead of calling
  do_perform_cow_encrypt (which is removed by that patch).

* Adjust qcow2_co_encdec to take full host and guest offsets as well.

* Document the qcow2_co_{encrypt|decrypt} arguments
  to prevent the bug fixed in former commit from hopefully
  happening again.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20190915203655.21638-3-mlevitsk@redhat.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[mreitz: Let perform_cow() return the error value returned by
         qcow2_co_encrypt(), as proposed by Vladimir]
Signed-off-by: Max Reitz <mreitz@redhat.com>